sound_bake in command line script
For Sound Drivers Addon I do a lot of sound baking, so I thought I'd check out how it would perform running in background mode (this has led to another q re running modal timer operators in bg mode). Anyhow using context override the following bakes in bg mode using command

blender -b ./Desktop/baketest.blend --python-text bake.py


using the following as a text block named bake.py.

import bpy
#from sound_drivers.utils import get_context_area
def get_context_area(context, context_dict, area_type='GRAPH_EDITOR',
                     context_screen=False):
    '''
    context : the current context
    context_dict : a context dictionary. Will update area, screen, scene, 
                   area, region
    area_type: the type of area to search for
    context_screen: Boolean. If true only search in the context screen.
    '''
    if not context_screen:  # default
        screens = bpy.data.screens
    else:
        screens = [context.screen]
    for screen in screens:
        for area_index, area in screen.areas.items():
            if area.type == area_type:
                for region in area.regions:
                    if region.type == 'WINDOW':
                        context_dict["area"] = area
                        context_dict["screen"] = screen
                        context_dict["scene"] = context.scene
                        context_dict["window"] = context.window
                        context_dict["region"] = region
                        return area
    return None

c = bpy.context.copy()
get_context_area(bpy.context, c)
obj = bpy.data.objects["Cube"]
obj["y"] = 0.0
obj.keyframe_insert('["y"]', frame=1)

speaker = bpy.data.speakers["Speaker"]
bpy.ops.graph.sound_bake(c, filepath=speaker.sound.filepath)
bpy.ops.wm.save_mainfile()


Note for convenience sake I grabbed the filepath from a speaker's sound.  This successfully baked the sound file onto the "y" custom property of the Cube in bg mode.

Use something like How to pass command line arguments to a Blender Python script? to extend functionality,.

NB: Example of creating a simple sound visualiser using sound_drivers.